GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed, cross-platform (supports Windows, Mac, and Linux) program for photo retouching, image composition, and image authoring. GIMP has many capabilities and it is expandable and extensible using plug-ins and extensions. You can certainly find some shortcomings of GIMP but for most non-professional people there is little Photoshop can do that cannot be done in GIMP.

Design blog Smashing Magazine has put together a fantastic list of eight tweaks for GIMP to make it a serious Photoshop contender. The list includes a plug-in that allows the use of Photoshop plugins, layer styles, CMYK color separation, Liquid Rescale, brushes, etc. If you can’t find what you need, you can check out the repository of available plugins or write your own.